The invention relates to a process and a system for production of dimethyl ether from synthesis gas in which at least one feed stream formed from synthesis gas (SG) is subjected to at least one synthesis step, in which components present in the feed stream are at least in part converted to dimethyl ether (DME), wherein at least one crude product stream is obtained which contains at least dimethyl ether (DME) and unreacted components of the feed stream, characterized in that the feed stream contains hydrogen, carbon monoxide and carbon dioxide corresponding to a stoichiometric number of 2.1 to 5.0 and contains 4 to 20 mol percent carbon dioxide, in that the ratio of carbon dioxide to carbon monoxide in the feed stream is in a range from 0.5 to 4, and in that the at least one synthesis step is carried out under isothermal conditions.
Dimethyl ether (DME) is the structurally simplest ether. Dimethyl ether contains two methyl groups as organic radicals. Dimethyl ether is polar and is conventionally used in liquid form as a solvent. Dimethyl ether can in addition be used as coolant, and replace conventional fluorochlorohydrocarbons.
Recently, dimethyl ether is increasingly being used as a substitute for fuel gas (liquid gas) and conventional motor fuels such as diesel. Owing to its comparatively high cetane number of 55 to 60, conventional diesel engines, for example, need only be modified slightly for operation with dimethyl ether. Dimethyl ether burns comparatively cleanly and without soot formation. If dimethyl ether is prepared from biomass, it is considered to be what is termed biofuel, and can therefore be marketed with tax advantages.
Dimethyl ether can either be generated directly from methanol, or indirectly from natural gas or biogas. In the latter case, the natural gas or biogas is first reformed to give synthesis gas. Synthesis gas can also be produced by means of other processes, for example by pyrolysis of waste or biomass. The synthesis gas is classically converted to methanol, and subsequently further converted to dimethyl ether. The production of dimethyl ether from synthesis gas is thermodynamically and economically advantageous compared with production from methanol.
The present invention relates to the single-stage or direct production of dimethyl ether from synthesis gas. A single-stage or direct production is here taken to mean production without an intermediate separation off of methanol, as proceeds in a two-stage production. The reactions that proceed, however, can also deliver methanol as an intermediate in a single-stage production, which methanol, however, further reacts at least in part to form dimethyl ether in the reactor or reactors used. Corresponding processes have been known for a relatively long time and are also described in more detail hereinafter.
US 2013/0030063 A1 relates to a process for the direct synthesis of dimethyl ether from synthesis gas in an isothermally operated reactor. Unreacted synthesis gas present in the reactor off-stream can be recirculated to the reactor. For this purpose, the reactor off-stream is first cooled, in such a manner that the predominant fraction of the water and of the methanol and also about 40% of the dimethyl ether condense out. The remainder of the dimethyl ether and the predominant fraction of the carbon dioxide are lashed by means of an absorber using methanol as absorbent. Finally, a recycle stream depleted in a complex manner in carbon dioxide and a fresh feed are fed to the reactor, where a stoichiometric number of the gas mixture reacted in the reactor is maximally at a value of 2.05 and the ratio of carbon dioxide to carbon monoxide thereof is maximally 0.25.
In the preparation of dimethyl ether, the Topsøe process which is considered in EP 1 026 141 A1 and described in more detail hereinafter can also be used. The Topsøe process and other processes for preparation of dimethyl ether are also mentioned, for example, in the article “DME—the new wonder fuel?”, Nitrogen & Methanol 260, 2002, pages 25 to 31.
There is still the requirement for a more flexible and more efficient process and systems for the production of dimethyl ether from synthesis gas.